A Ground-Motion Model Derived Using a Generalized Mean Rupture Distance for Large Slab Interface Earthquakes

Abstract

Source–station distance is a central input to ground-motion models (GMMs) for predicting seismic shaking. GMM development uses distance metrics including the Joyner–Boore distance, which is the shortest distance from an observation point to the surface projection of the earthquake rupture, and Rrup the shortest distance to the rupture in three dimensions. Thompson and Baltay (2018) proposed the generalized mean rupture distance Rp to address observed near-fault ground-motion saturation. Rp accounts for the contribution to the shaking of all parts of the rupture and provides a simple method for incorporating spatially variable slip. They used Rp to develop a GMM for shallow crustal earthquakes, assuming uniform slip. Here, we investigate the improvement offered by an Rp-based GMM for large subduction interface earthquakes by recalibrating the path term for a published GMM (Parker et al., 2022) using Rp derived from distributed slip models (DSMs). Inspection of within-event and total residuals indicates that the recalibrated model fits the data at least as well as an alternative in which the path term was recalibrated using Rrup and the same dataset. Incorporating slip information in its entirety or trimming the DSM geometry is preferable to assuming uniform moment release on a prescribed model fault that extends beyond the actual ruptured area. Rp tuned to minimize model uncertainty is closer to the maximum distance to rupture for peak ground velocity (PGV) than acceleration, possibly reflecting the contribution to high-frequency shaking of slip on local asperities. However, when the moment is concentrated far from stations, PGV is fit adequately with Rp closer to Rrup. Our results suggest incorporating slip-derived moment release through Rp could improve GMMs for slab interface events, especially if the implementation of Rp-based models is refined using a larger dataset of earthquakes with greater geographic diversity to account for regional ground-motion variations.